The invention relates to a gas sensor comprising a sintered metal oxide semiconductor and adapted to sense gas leakages of natural gas, liquefied petroleum gas such as methane, butane or propane, namely alkane gases by means of a change in an electric resistance of the semiconductor per se and/or a wire extending therethrough.
Natural gases and liquefied petroleum gases are widely used as fuels for domestic use and industrial uses. There are utilized gas leakage alarming devices made of materials such as sintered metal oxides, for example tin oxide (SnO.sub.2) or zinc oxide (ZnO) employed to function as sensing elements of the alarming devices.
The known sensing elements comprising the sintered metal oxide semiconductors are however not so selectively sensitive to methane that the alarming devices detect other miscellaneous gases such as hydrogen, carbon monoxide, vapor of alcohol (in particular, ethanol), and cigarette smoke at the same sensitivity as in the case of sensing methane.
Consequently, the known alarming devices are disadvantageous in that they are apt to malfunction due to alcohol vapor liberated from alcoholic liquors used in home cooking or due to sprayed insecticide solution. The other known gas leakage alarming devices equipped in factories or plants are also likely to generate malfunctional alarms caused by exhaust gases emitted from running automobiles or neighboring other factories. It has therefore been tried to solve the above problem, i.e. such a poor selectivity in sensing one of the aforementioned gases which is to be detected to generate an alarming signal. In case of detection of methane gas, it is proposed to coat the sensing element with a layer of one of precious metal catalysts such as platinum and palladium. However, the layers of these precious metals are not only expensive but also considerably deteriorative.